This application claims the benefit of Japanese Patent applications Nos. 11-32900 and 2000-1968 which are hereby Incorporated by reference.
1. Field of the Invention
The present invention relates to a variable focal length optical system and, in particular, relates to a variable focal length optical system suitable for use as a high zoom ratio lens.
2. Related Background Art
Since portability has recently been regarded as important for a lens shutter type camera, a small-sized lightweight picture-taking lens is proposed. It has recently become common to use a zoom lens in a picture-taking lens used for a lens shutter type camera. A zoom lens has a merit for a photographer to make it possible to take picture at will in accordance with variation of focal length. On the other hand, it also has a demerit to make it difficult to make a camera body being compact since total length of a lens system varies drastically according to variation of its focal length. In order to reduce size and weight of a camera body, it is inevitable to reduce size of a picture-taking lens and, therefore, it is important to reduce total length and diameter of a picture-taking lens.
In order to reduce the total lens length, it is suitable to strengthen the power of each lens group consisting of the optical system. However, since the refractive power of each lens surface becomes strong, the optical quality of the optical system will be degraded. In order to reduce diameter of an optical system, it is suitable to reduce the number of lens use in the optical system. Since, in this case also, the refractive power of each lens surface becomes strong, the optical quality of the optical system will be degraded.
Further, since production technology of aspherical lens is significantly improved lately, it has become common to accomplish compactness by positively putting aspherical lens into picture-taking lens. When the aspherical lens is arranged in the vicinity of the aperture diaphragm, it will be served as correcting spherical aberration. When an aspherical lens is arranged away from an aperture diaphragm, it will be served as correcting off-axis aberration. The degradation of the optical quality caused by getting stronger the refractive power of each lens surface is reduced by aberration correction function of an aspherical surface. Therefore, it is accomplished that the picture-taking lens is made to be small-sized.
An aspherical lens is classified roughly into a single side aspherical lens and a double aspherical lens. Although single side aspherical lens is common, it is proposed that both sides aspherical lens is introduced in a variable focal length optical system, such as Japanese Laid-Open Patent Application Nos. 3-127012 and 4-78811.
In the case developing a zoom lens for a lens shutter type camera having a high zoom ratio and high optical quality maintaining compactness, if further high specification or further compactness is required, it is necessary to further reduce the number of lens or to make the refractive power of each lens surface stronger. Therefore, it is necessary to make use of more aspherical lenses. Since aspherical lenses are mainly made by molding process suitable for mass production, the response in high spatial frequency is not linear enough, because of a minute undulatory element remained on the surface of metal mold for molding In other words, there is a problem that the output deforms with respect to input. Therefore, compactness cannot always be effectively accomplished by simply increasing the number of aspherical lens. Accordingly, it becomes important to effectively make use of smaller number of aspherical lenses.
In variable focal length optical systems disclosed in Japanese Laid-Open Patent Application Nos. 3-127012 and 4-78811, the aberration correction function of each aspherical surface is not clearly allocated. Further, it is problem that the effect of compactness and high quality resulted from introduction of aspherical lens is not sufficient. Since a lot of aspherical lenses are used in those lens systems, decentering or the like introduced in the manufacturing process degrades optical quality. Therefore, there is another problem that the product (optical system) with stable optical quality cannot be provided to customer.
The present invention is made in view of the aforementioned problems and has an object to provide a variable focal length optical system suitable for reducing the size and increasing the zoom ratio of the system.
According to one aspect of the present invention, a variable focal length optical system includes at least three lens groups, in order from object side, a first lens group G1 having a positive refractive power, a second lens group G2 having a positive refractive power, and a third lens group G3 having a negative refractive power. When a state of lens group positions is changed from a wide-angle end state to a telephoto end state, each lens group moves to the object side such a manner that an air gap between said first lens group G1 and said second lens group G2 increases, and an air gap between said second lens group G2 and said third lens group G3 decreases. An aperture diaphragm is arranged between said first lens group G1 and said third lens group G3. Said second lens group G2 is arranged in the vicinity of said aperture diaphragm and has a positive lens having double aspherical surfaces. The following conditional formulae are satisfied:
Da/f less than 0.08xe2x80x83xe2x80x83(1)
0.3 less than (Ra+Rb)/(Raxe2x88x92Rb) less than 0.95xe2x80x83xe2x80x83(2)
where Da denotes an air gap along to an optical axis between said aperture diaphragm and said positive lens, f denotes a focal length of said variable focal length optical system, Ra denotes a radius of curvature of a surface of said positive lens facing to a space where said aperture diaphragm exists, and Rb denotes a radius of curvature of a surface of said positive lens facing to a space opposite to said space where said aperture diaphragm exists (where Rb less than 0).